This invention relates in general to vehicle suspension systems and, in particular, to a fluid suspension system especially useful for a two-wheel tractor.
More specifically, but without restriction to the particular use which is shown and described, this invention relates to a fluid suspension system used in a two-wheel tractor and two-wheel earthmoving scraper combination, neither of which is capable of independent level support.
Tractor scrapers are used in many construction applications, such as road building, wherein these four-wheel, two-axle vehicles are used to transport heavy loads of material such as dirt, rock, and gravel over rough terrain from one location to another for levelling or filling the terrain. The scraper comprises a two-wheel tractor and a two-wheel earthmoving trailer which are articulated about a combined draft and steering coupling between the tractor and trailer. The two units are interconnected by a yoke attached to the scraper and pivotally mounted on the tractor by a king pin coupling. The tractor scraper is steered by turning the tractor portion, with respect to the trailer, about the vertically disposed articulation pivot axis through the use of hydraulic jacks connected between the two portions of the vehicle.
In the operation of such two-axle earthmoving tractor scrapers, the resiliency of the tires and the weight of the scraper trailer suspended between the front and rear wheels of the combination creates a vertical bouncing or pitching movement especially during hauls at relatively high vehicle speeds. Due to the large size and weight of the equipment, and of the loads transported by it, ordinary shock absorbers or vibration isolators are impractical to resolve the problems occurring due to such bouncing or pitching movement. Such bouncing or pitching movement is uncomfortable and potentially dangerous for the machine operator and, due to the large shock loads imparted by the bouncing of the trailer unit, can result in damage or failure to structural parts of the vehicle, particularly those closely associated with the load carrying body.
Various attempts have been made to resolve or control bouncing or pitching in tractor-trailer combinations. For example, there is disclosed in U.S. Pat. No. 3,321,216 a method and apparatus for controlling such bouncing through the use of a secondary vibration system tuned to cancel the undesirable vibrations in the primary system.
Another attempt to minimize such bounce is disclosed in U.S. Pat. No. 3,135,529 wherein the tractor portion of the tractor scraper is supported by means of liquidsupport devices. These devices are suspended from the vehicle frame so that the tractor portion is supported by the pressure exerted on the liquid by the weight of the trailer portion of the vehicle. Other such devices are positioned between the tractor axle and the tractor frame to further assist in cushioning the shock and bouncing rhythm.
Another attempt to dampen this bouncing or pitching movement is disclosed in U.S. Pat. No. 3,680,892 which describes an automatic levelling or control device utilizing a mechanical linkage to transmit relative motion between the hitch components to a control valve for a hydraulic cylinder utilized to cushion the vertical movement between the tractor and scraper.
U.S. Pat. No. 3,845,833 discloses another manner of dampening the vertical movement or "lope" of the vehicle by utilizing a pair of ride cylinders, one on each side of the vehicle, which are pivotally mounted between the tractor axle and frame. A similar suspension system is disclosed in U.S. Pat. No. 3,827,518 which utilizes a pair of links permitting the tractor to pitch about a transverse axis as well as permitting relative vertical movement between the tractor and trailer unit. A subframe is pivotally connected to the steering frame and air-oil spring devices are used for cushioning the bounce and pitch.
While many attempts have been made to resolve the problems associated with bouncing and pitching of a tractor scraper, such systems have generally involved complicated hydraulic compensation systems and complex linkage which, in the event of a malfunction, add to the problems these devices were intended to solve. In addition, the complexity of some systems increases unit cost due to the required additional components and decreases the reliability of the scraper equipment which must be shut down in the event of system malfunctions.